This invention relates to evaluation of salt sensitivity as it relates to propensity to develop hypertension.
It is estimated that 25% of the U.S. population is salt-sensitive, responding to high salt intake with an increase in blood pressure. 25-Hydroxyvitamin D, a liver metabolite, is the precursor of the hormonal form of vitamin D, 1,25-dihydroxyvitamin D (1,25-(OH)2D), which is synthesized in the kidney. Previously it was demonstrated that an inverse association can be found between plasma 25-hydroxyvitamin D (25-OHD) concentration and blood pressure in Dahl salt-sensitive (S) rats during high salt intake. Plasma 25-OHD concentration of S rats decreased with days fed a high salt diet, as blood pressure increased. Spontaneously hypertensive rats do not have low plasma 25-OHD concentrations, suggesting that reduction of plasma 25-hydroxyvitamin D concentration may be specific to salt-induced hypertension.
It is also now known that exogenous 25-OHD will not attenuate the blood pressure increase in S rats fed a high salt diet, nor will exogenous 25-OHD increase plasma 25-OHD concentration, suggesting a higher rate of metabolism and/or clearance of 25-OHD when S rats are fed a high salt diet.
Elderly hypertensive females with low plasma renin activity, characteristic of salt-induced hypertension, were reported to have significantly lower plasma 25-OHD concentrations compared with normotensive elderly and young females. Black Americans have a higher rate of salt-sensitive than white Americans, and a higher rate of hypertension. Black Americans have been shown to have significantly lower mean plasma 25-OHD concentrations compared with white Americans in several studies involving both males and females and in a report based on the third National Health and Nutrition Examination Survey (NHANES III, 1988-91). The NHANES III study comprised 18,875 adolescents and adults. Prevalence of low 25-OHD values (less than 10 ng/ml) was greater in non-Hispanic blacks than in non-Hispanic whites.
Low levels of 25-OHD in black Americans have been ascribed, in part, to reduced epidermal vitamin D photosynthesis associated with high melanin skin content. The lower mean plasma 25-OHD concentrations found for black subjects in the studies might also be affected by lower plasma 25-OHD concentrations in a subset of salt-sensitive black subjects with borderline or moderately high blood pressure. In a study of matched pre-menopausal females without history of hypertension, mein serum concentration of 25-OHD was slightly, but not significantly, lower in the 70 black subjects compared with the 67 white subjects. Young Dahl S rats fed a low salt diet exhibit plasma 25-OHD concentrations slightly, but not significantly, lower than that of R rats.
Based on the blood pressure change in response to a salt load, a previous report found 18% and 37% salt sensitivity for 18-23 year-old Caucasian and African-American subjects, respectively. Another report found 22% prevalence of salt sensitivity in 140 African-American adolescents.